1. Field of the Invention
This invention relates to semiconductor dies, also commonly referred to as xe2x80x9cchips.xe2x80x9d It is particularly directed to the fabrication of a capacitor on a semiconductor die, the term dice including not only singulated dies but also partial wafers and entire wafers from which dies may be separated.
2. State of the Art
Circuit boards and other carrier substrates or platforms, both single- and multi-level, are employed to mechanically support and electrically connect both active devices, such as semiconductor dies, as well as passive components, such as resistors, capacitors and inductors. Conventional chip on board (C.O.B.) assemblies typically include discrete components of both classes, e.g., resistors, capacitors, and inductors, as well as various solid state xe2x80x9cchipsxe2x80x9d or xe2x80x9cdiesxe2x80x9d hard wired, plugged, soldered, ball-bonded or otherwise electrically connected to a conductor network carried by a platform such as a card, board or other substrate. In the prior art, discrete capacitors as well as resistors and inductors are often required to be placed on the platform in combination with adjacent, associated semiconductor dice, for example in the construction of a single in-line memory module (SIMM). With the increased circuit densities required by ever-faster processors and larger memories, due to a generally fixed board or platform area, commonly termed xe2x80x9creal estatexe2x80x9d in the semiconductor industry, the use of discrete passive components is viewed as an undesirable but hitherto necessary waste of real estate which could otherwise be applied to better and higher uses. The only exception to the requirement for a discrete capacitor known to the inventors is the fabrication of a capacitor on the board to which chips or dies may be attached.
It should be noted that the discrete capacitors referenced above are chip- or die-level capacitors serving as protective, noise-reducing gateways between the die and the remainder of the circuit in the assembly. Such capacitors are to be distinguished from the cell capacitors employed in dynamic random access memory (DRAM) cells, each cell of which includes a transistor (MOSFET) and capacitor. U.S. Pat. Nos. 5,547,063; 5,457,065; 5,459,094 and 5,459,095 illustrate various constructions which provide capacitance for dynamic random access memory (DRAM) cells. In each instance, the capacitor construction is integrated into the cell itself. As noted, such capacitors simply provide the storage requirement of the cell. U.S. Pat. No. 5,461,536 similarly discloses the construction of planar storage capacitors in association with the cells of a DRAM chip. The method disclosed involves a sequence of deposition and etching steps which add both electrode and dielectric layers to chips constructed from inorganic oxides.
Semiconductor chips or dice are conventionally fabricated by a series of material deposition, removal and conversion steps to selectively add, remove and alter the state of silicon and various metals to form the end-product die. For example, materials may be deposited upon, removed from, or converted to a different structure on a region of a substrate, either to form conductive structures on that level (such as circuit traces or bond pads), to form insulative, dielectric or passivating structures, or to penetrate beneath and form inter-level conductive elements (such as contacts or vias) or active component segments (n- and p-doping by diffusion or ion implantation) of the semiconductor devices residing in the die. Stated simplistically from another perspective, a semiconductor die may be viewed as a stack of layers, any of which may function as a substrate for adjacent layers. Typically, layers containing active (transistors, diodes, memory cells) or passive (capacitors, resistors) electronic components are separated by dielectric layers to avoid electrical shorts.
There remains a need for a simple, low cost technique for incidentally constructing a capacitor on or within a die to replace the chip capacitor of a SIMM board or other assembly having a comparable need for a capacitive or other passive component in association with the die, and supplied in the prior art by a discrete structure connected to the same platform as the die.
According to the present invention, a capacitive element is fabricated directly as a layer, ideally the top conductive layer, of a semiconductor die, such as a dynamic random access memory (DRAM) chip. This construction approach avoids the necessity commonly confronted in C.O.B. technology for a capacitor chip (or equivalent discrete component) to be connected, as by solder paste or conductive epoxy, to the circuit board or other platform incorporating the die. Thus, each DRAM or other chip structure supported on a conductor. platform, such as a circuit board, is provided with an integral die-level gateway capacitor as well as a resistor, if required, for special applications.
The die-level gateway capacitive element of the invention may be constructed through conventional mask (positive or negative resist) techniques in an additive or subtractive manner and in single or parallel planes. Ideally, the organic die coat which may be conventionally present on a finished die is relied upon to function as the dielectric component of the capacitor. Some die fabricators employ such a die coat in all instances, while others employ a die coat only with an LOC (leads over chips) configuration. The die coat composition may be selected specifically for its dielectric properties. Certain preferred embodiments include other components, such as a resistor, in the mask pattern. In this fashion, pre-designed capacitance and resistance values can be furnished in the die itself.
Preferably, the gateway capacitor is formed on the upper, front or active surface of a die. However, it is sometimes permissible or preferred to form the capacitor on the normally unused back surface of the die or incorporate it within one or more layers within the die itself.
Placing the gateway capacitor on the die eliminates the need for capacitor-to-die traces on the circuit board required with prior art discrete decoupling capacitors. Noise generated by the circuitry, as xe2x80x9cseenxe2x80x9d by a chip, is reduced to the lowest possible level by locating the capacitor as the final barrier between the circuit board and the chip. Chip performance is thereby optimized. Moreover, forming a capacitor in this fashion may require no additional steps when done in a die fabrication process already involving terminal or bond pad construction. All that is required is to utilize an appropriately modified mask for the fabrication process, unless a die coat not normally on the die must be added. The capacitor may be laid out and constructed incidental to the construction or rerouting of other conductive elements during the fabrication process on any convenient layer of the die having an adequate area to accommodate the capacitor traces and intervening dielectric. For example, rerouting peripheral bond pads into an array pattern for flip-chip applications presents a design opportunity to incorporate a capacitor according to the invention.
Positive and negative, generally planar, laterally-extending capacitor electrodes may be formed on the die surface or other layer in any convenient pattern effective to achieve a desired capacitance, typically less than about 500 pF, and usually within the approximate range of about 100-200 pF. One suitable such pattern positions the electrodes in a single-layer, side-by-side serpentine configuration with a narrow dielectric space or region laterally separating the electrodes. The capacitor of this invention can alternatively be formed in a more traditional configuration with two facing conductive layers separated by a thin dielectric layer. This configuration requires less real estate on the die, but is otherwise generally not preferred, because it may require additional steps in the fabrication process to deposit the intermediate dielectric layer, as well as a second conductive layer.
The capacitor may be positioned any place on or beneath the surface of a chip to accommodate the terminal pattern necessary for that particular chip. If placed on the top surface of the die, a passivating layer, such as polyimide, silicon nitride, silicon dioxide, BPSG, or a combination of any of the foregoing or other suitable materials known in the art, may be placed thereover for physical protection and electrical isolation. The invention thus has wide flexibility within its characteristic capacitance range. In any event, the electrodes are generally planar in the preferred embodiments. That is, the surface area of an electrode parallel the die layers is many times, typically orders of magnitude, greater than the surface area of the electrode edges as viewed generally perpendicular to the die layers.
It is contemplated that air, e.g. an unfilled space between laterally adjacent capacitor electrodes, may serve as the capacitor dielectric and the term xe2x80x9cdielectric,xe2x80x9d as used herein, encompasses an inter-electrode air gap.
The capacitor will ordinarily include positive and negative bond pads, which may be located as required to facilitate bonding to a circuit board. These pads should be positioned at an edge of the chip in embodiments intended to be wire bonded to a circuit trace on a circuit board in order to employ the shortest possible bond wire lengths. Alternative constructions may position the bond pads remote from the perimeter of the die as convenient, e.g., for flip chip applications. The flip chip fabrication process may result in a matrix of relatively large bond pads (to accommodate the bump-type bonds employed) in the middle of the die. The configuration of the capacitor may then be adjusted appropriately, as by intertwining it around the various elements, e.g., pads, other terminals, or vias, on or extending through the layer or layers of the die upon which the capacitor is fabricated. In a flip-chip arrangement, for example, with a large centralized matrix or pattern of bond pads, the capacitor may extend peripherally on the die, extending around the bond pad matrix and electrically connecting to two of the bond pads by inwardly-extending conductors.
While a subtractive etching process employing either negative or positive photoresist to selectively remove a previously-deposited conductive layer is presently preferred to form the traces for the capacitor as well as integral conductors leading to bond pads at the distal ends, a deposition or additive process of depositing the capacitor electrode structure and associated conductors through a pre-placed mask could also be employed for this purpose. It is also generally preferred, as noted above, that an outer dielectric layer be applied over the capacitor traces on a die surface to prevent shorting and to protect them from physical damage.
Conventional die fabrication typically involves applying a preselected quantity of a die coat material, such as liquid polyimide or a polyimide precursor, on the top of a wafer. The wafer may then be spun to distribute the coating material evenly about the wafer""s surface. The coating is then allowed to harden. According to this invention, after the coat is hardened, a conductive layer, such as aluminum, an aluminum alloy, copper, or tungsten, is placed over the die coat. The conductive layer may be etched after masking to form the capacitor traces and the terminals for the chips to be formed from the wafer in a single step. It is generally convenient to use the same material, typically aluminum or aluminum/copper alloy, for both the capacitor electrodes and the die bond pads. It is important to match the coefficient of thermal expansion of the capacitor materials to that of the rest of the die to avoid thermally-induced delamination as the chip heats and cools in operation. Thermal expansion considerations also influence the pattern selected for the capacitor electrodes. A serpentine pattern is often preferred from this standpoint. A die will ordinarily tend to expand from its center of mass, and expansion is minimized by providing a shorter trace in any given direction, as is inherently provided by a serpentine or comparable configuration.